Fabrication and Chacterization of an All-Solid-State Stretchable Supercapacitor

using Polypyrrole-CNT Hybrid Partially Embedded in PDMS

R. Zhang, J. Xu and E. H. Yang

*Stevens Institute of Technology, Hoboken, NJ, USA, rzhang22@stevens.edu

ABSTRACT

We present a supercapacitor with polypyrrole-

dodecylbenzenesulfonate (PPy(DBS))-vertically aligned

carbon nanotube (VACNT) hybrid partially embedded into

PDMS. The facile technique enables the partial-embedding

of vertically aligned carbon nanotubes in PDMS, which

facilitates a stable charge/discharge under varied strains.

The measured capacitance was 2 mF/cm² at a scan rate of

1000 mV/s with a gel electrolyte. The structure was

seamlessly stretched up to 160% with a capacitance

attenuation by 30% at 160% stretching. The measured

capacitance valuse were very well maintained under

bending with its angles varying from 0 to 180 degrees.

Keywords: flexible supercapacitors; vertically aligned

carbon nanotubes; polypyrrole-dodecylbenzenesulfonate;

PDMS; fabrication process

1 INTRODUCTION

Energy storage technologies can be categorized, based

on the form of energy stored in the system, into mechanical

(pumped hydroelectric storage, compressed air energy

storage and flywheels), electrochemical (conventional

rechargeable batteries and flow batteries), electrical

(capacitors, supercapacitors and superconducting magnetic

energy storage), thermochemical (solar fuels), chemical

(hydrogen storage with fuel cells), and thermal (sensible

heat storage and latent heat storage) energy storage.

Supercapacitors can be categarized as electric double

layer capacitors, pseudocapacitors, hybrid capacitors

(including electric double layer capacitors and

pseudocapacitors). Applying these supercapacitors into

flexible electronics would benefit a wide range of

applications in wearable and multifunctional electronics,

including flexible displays, curved smart phones, electronic

skins, and implantable medical devices. More specifically,

flexible supercapacitors can meet the demand of light-

weight, flexible, and portable electronic devices.

Consequently technologies for flexible energy storage have

to be developed for such flexible electronic devices [1,4,5],

and efforts have been made to improve the performance of

the flexible supercapacitors. Among several candidates for

supercapacitor materials, vertically aligned carbon nanotube

(VACNT) and polypyrrole-dodecylbenzenesulfonate

(PPy(DBS)) are promising materials for electrodes of

flexible supercapacitors owing to their excellent electrical,

and mechanical properties [6–10].

In this work, we present a supercapacitor composed of

PPy(DBS)-VACNT hybrid, partially embedded into PDMS.

The device is facilely fabricated by the partial-embedding

of PPy(DBS)-VACNTs in PDMS, permitting a strong hold

of PPy(DBS)-VACNT hybrid into Polydimethylsiloxane

(PDMS), which facilitates a stable charge/discharge under

varied strains.

2 EXPERIMENTAL SECTION We synthesized VACNTs using atmospheric-pressure

chemical vapor deposition (APCVD) into carpet-like

structures. The catalyst layer consisting of 5 nm Al and 3

nm Fe was deposited on the Si/SiO₂ substrate prepared

using physical vapor deposition (PVD). Then the substrate

was placed in the atmosphere pressure chemical vapor

deposition (APCVD) chamber. The furnace temperature

was increased to 750˚C with a constant 500 sccm Ar flow.

VACNTs were grown at 750˚C for 15 minutes with 60

sccm H₂ and 100 sccm C₂H₄. Then the chamber was cooled

down to the room temperature while maintaining the same

Ar flow rate. The structure of the grown CNTs is vertically

aligned in general, while the individual carbon nanotubes

are entangled with each other. The morphology of the

71Materials for Energy, Efficiency and Sustainability: TechConnect Briefs 2018

VACNTs were characterized using scanning electron

microscopy (SEM).

To fabricate flexible substrates with embedded

VACNTs, we transferred the grown VACNTs onto partially

cured PDMS. First, we used a liquid mixture of PDMS base

and curing agent (Sylgard 184 Silicone Elastomer, Dow

Corning) which were mixed with a ratio of 10:1 to form a

PDMS substrate. After degasing under reduced pressure in

a vacuum pump, the bubbles were all removed while

PDMS was still liquid. Then the liquid PDMS was placed

on a hot plate at 65˚C for about 30 minutes before it was

fully cured. We optimized the curing condition of PDMS,

where the partially cured PDMS was tacky but not fully

wet. The grown VACNTs were then placed onto partially

cured PDMS. Then the tips of CNTs were partially

immersed into PDMS slowly. During the curing process,

the embedded CNTs were eventually wetted by PDMS.

Then after PDMS was fully cured, the VACNT-PDMS

structure was successfully peeled off from the Si/SiO₂

substrate owing to the strong adhesion between PDMS and

VACNTs. The entire fabrication process is rapid and facile,

which permits the integration of VACNT-PDMS substrate.

After VACNT embeding, PPy(DBS) film was

electropolymerized atop the VACNT-covered substrate.

First, 1mL pyrrole monomer (reagent grade, 98%, Sigma-

Aldrich, St. Louis, MO) was thoroughly mixed with 150mL

0.1mol/L sodium dodecylbenzenesulfonate (NaDBS,

technical grade, Sigma-Aldrich, St. Louis, MO) solution.

Then, VACNT-covered substrate, a saturated calomel

electrode (SCE, Fisher Scientific Inc., Pittsburgh, PA), and

stainless steel mesh (5 cm × 5 cm) were submerged in the

solution as the working, reference, and counter electrode,

respectively. The coating of PPy(DBS) surface was carried

out using a potentiostat (263A, Princeton Applied Research,

Oak Ridge, TN) by applying 0.8V to the working electrode

(vs. SCE) and stopped once surface charge density reached

300 mC/cm2. After fabrication, PPy(DBS) surface was

rinsed and dried in air overnight before any further

characterizations.

PVA-KOH gel electrolyte was fabricated by mixing 1g

poly(vinyl alcohol) (PVA) powder with 1g KOH powder in

10 ml DI water. After mixing at 60˚ for around 1 hour and

continuously stirring, it took on a homogeneous viscous and

clear appearance. Then PVA-KOH gel electrolyte was

dripped onto the surface of the VACNT-PDMS structures.

The all-solid-state flexible supercapacitor was fabricated by

stacking two PPy-VACNT-PDMS structures face-to-face

with PVA-KOH gel electrolyte in the middle. The

electrochemical property of the flexible supercapacitor was

measured at different scan rates from 50 mV/s to 1000

mV/s using cyclic voltammetry.

The electrochemical behavior was characterized using

cyclic voltammetry (CV) in a two electrode configuration.

CV measurements were performed within the potential

range of 0.0V-0.5V as scan rates of 50-1000 mV/s by using

a potentiostat. The capacitances of the electrodes were

calculated as a capacitance per area (F/𝑐𝑚2). The average

capacitance was normalized per area of the samples and

was estimated according to the following equation [11–13]:

𝐶 =∫ 𝐼𝐸2𝐸1

𝑑𝑉

∆𝑉×𝑉×𝐴 (1)

where I is the current, A is the area of the supercapacitor,

∆V is the scanning rate, E₁ and E₂ are the voltage and 𝑉 =

𝐸2 − 𝐸1.

We performed both the tensile strain measurements and

the bending strain measurements by using a stretching stage

manually to evaluate the flexibility and durability.

3 RESULTS AND DISCUSSION

Figure 1 shows the cyclic voltammetry (CV) curve

recorded at a scanning rate of 1000 mV/s with PVA-KOH

gel electrolyte. As can be seen, the structure showed good

electrochemical stability and capacitive behaviors at

scanning rate of 1000 mV/s. The capacitance can be

calculated to be 2 mF/cm² at 1000 mV/s.

72 TechConnect Briefs 2018, TechConnect.org, ISBN 978-0-9988782-3-2

Figure 1: Cyclic voltammetry curve of the flexible

supercapacitors at a scan rate of 1 V/s.

Figure 2: Galvanostatic charge/discharge curve at the

current density of 0.1875 mA/cm².

4 CONCLUSION

We have demonstrated a flexible supercapacitor

composed of PPy-VACNTs on PDMS substrate. The

device was fabricated using a new, facile technique that

enables the partial-embedding of vertically aligned carbon

nanotubes in PDMS. As a result, the devices were reliable

and stable under both stretching AND bending (flexibility)

for a long-cyclic testing. The fabrication process was very

reproducible and reliable. As next steps, the performance of

this flexible supercapacitor will be fully characterized at

various applied strain values (stretching, bending and

twisting at different frequencies under different

temperatures and humidity values). The cyclic behavior of

supercapacitors will be investigated under such applied

strains. The droplet behavior of the surface of PPy-

VACNT-PDMS structure will be characterized under PPy

redox and under different stretching strains at varying PPy

thickness and surface morphology.

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